1. Field of the Invention
The present invention relates to digital radiology and, more particularly, to a large array, single exposure digital mammography machine. The digital mammography machine may process, store access, and transmit data in the same manner as other types of digital image data.
2. Discussion of the Prior Art
According to the American Cancer Society, in 1995 182,000 women will be diagnosed with breast cancer and 46,000 women will die from breast cancer. The ACS estimates that 2,000,000 women will be diagnosed and more than 500,000 women will die of breast cancer in the 1990's. Early breast cancer detection increases the patient's chances of surviving the cancer. Thus, early detection is a major factor in saving the lives of breast cancer patients. Experts agree that a mammogram is the single best means of early breast cancer detection.
Mammography is the radiological examination of the human breast. It is generally accepted that mammography is an effective and reliable procedure in the early detection of breast cancer. Mammography is typically performed using x-ray or other traditional film/screen techniques. However, these techniques do not always provide adequately high-quality images to detect cancer, particularly in the relatively large population of women having radiodense breast tissue (younger women, for example, tend to have radiodense breast tissue). Mammograms require high-quality images because the tissue density between adipose (fatty), glandular, calcified, or cancerous tissue is less diverse than, for example, flesh and bone. Thus, "subtler" contrasts are desirable to distinguish between these types of tissue. Traditional film mammograms have a non-linear response to x-ray exposure. That is, for example, doubling the x-ray exposure of film or halving the breast density, does not result in an image that is twice as bright. As a result, a single traditional film x-ray exposure often do not show the entire tonal range of a patient's breast tissue. Often, a radiologist may take exposures at different energy levels to provide images with different contrasts. This exposes the patient to several doses of x-rays.
Other drawbacks are caused by the poor contrast of film mammograms. One of these drawbacks is that it is difficult to detect masses in patients', having breast implants. A second drawback is that it is difficult to discern between benign and malignant microcalcifications and tumors. This latter drawback results in thousands of unnecessary invasive procedures to remove growths which are later determined to be benign. If a mammogram could allow a radiologist to distinguish more clearly between benign and malignant tissue, many of those procedures would be prevented.
FIG. 1 illustrates a conventional mammography machine using a traditional film technique. The conventional mammography machine 50 has an x-ray tube 52 which emits x-rays and an image receptor 54 which receives the x-ray radiation. During use, a breast 56 is compressed between a compression plate 58 which holds the breast in position, and a bucky tray 60, which sits on top of the image receptor, supports the breast 56 and houses a grid 62. Beneath the grid is a scintillator 64 which generates visible light when excited by x-rays. A film 66 is placed below the bucky tray and scintillator and on top of a radiation detector 68 used for automatic exposure control, such as an ionization chamber or a solid state sensor. X-ray radiation passes through the breast tissue and strikes the scintillator 64. The scintillator generates visible light according to the x-ray striking it. The visible light enters the image receptor and exposes the film. Areas where the x-ray radiation does not pass through the tissue result in a light area on the film indicating a mass or other x-ray blocking body. After the film is exposed, it is developed before it may be viewed.
Digital mammography which employs a solid state electronic imager in place of film is an emerging technology in the detection of breast cancer. Digital mammography has advantages over the traditional types of mammography. The National Cancer Institute recently sponsored workshops on breast imaging and digital mammography. One conclusion from these workshops is that digital mammography has great potential for use in the diagnosis and screening of breast cancer and for use during invasive procedures on breast cancer patients. Digital mammography offers:
1. Shorter procedure times, resulting in improved patient comfort and less likelihood of image blurring due to patent motion (including involuntary motions such as heartbeats and breathing); PA1 2. improved image quality due to (a) larger dynamic range and linear x-ray exposure response (linear latitude) of electronic imaging techniques, and (b) computer-aided visualization. This improved image quality permits clearer imaging of patients having breast implants and a greater likelihood of distinguishing between benign and malignant growths; PA1 3. decreased healthcare costs because digital mammograms provide early detection; early breast cancer detection typically results in lower treatment costs. Also, the improved image may reduce the number of unnecessary procedures because of potentially better visualization of suspicious morphology; and PA1 4. lower radiation exposure because of the lower radiation-per-image requirements for the shorter exposure time, and the resultant accurate images reduce the need to re-take an image or take several exposures.
Digital mammography currently has several shortcomings. One major shortcoming is that most known digital mammography machines cannot take an image of an entire breast in one exposure. One reason for this is because the electronic imagers for capturing the image are small and cannot be arranged together to provide a gap free imaging surface large enough to image an entire breast in a single exposure. One well known electronic imager is a charged coupled device (CCD). FIG. 2 illustrates a typical CCD 100. A CCD is typically a device having an imaging surface area 102 no larger than 1".times.1" and containing up to 2000.times.2000 pixel elements. This one square inch area is not sufficient to image an entire breast. Current CCD technology uses at least one side of the CCD 100 for circuitry 104 for peripheral components, such as amplifiers and serial registers. These peripheral components on the side of the CCD 100 prevent CCDs from being butted together on four sides without a significant gap in the imaging surface. Thus, the first digital mammography machines were small devices used during invasive procedures to locate suspicious masses during needle localization or core biopsy procedures. The small imaging surface of CCDs has prevented widespread use of digital mammography.
Digital mammography also faces severe technical challenges. Soft-tissue breast imaging has the most stringent imaging requirements of all radiological imaging. Two reasons for this are the slight difference in densities between the tissue types found in breasts (adipose, glandular, calcified, and cancerous) and the relatively small size of breast tumors in their initial stages. As a result, mammography requires very fine pixel dimensions (e.g., less than 40 microns) and a high contrast dynamic range (14 bits, i.e., 2.sup.14 or 16,384 tone levels) over a large area (i.e., 24 cm.times.30 cm). The American College of Radiology recommends an image resolution of 11 to 13 line pairs per millimeter. This means that the image should be sharp enough to distinguish between 11 to 13 pairs of white and black lines in a one millimeter space.
Several digital mammography machines are commercially available. Because of the large number of pixels needed to image an entire breast and electronic detector size limitation described above, these mammography machines use one of two systems to image an entire breast.
The first system is a scanned beam system. In a scanned beam system, the electronic imager is mechanically scanned along the patient's breast as beams of radiation are transmitted towards the imager at each position, until the entire breast is imaged. The x-ray beam is collimated into a long thin line and scanned slowly across the breast and a thin, linear scintillator-coupled CCD array is synchronously scanned beneath the breast in perfect registration with the beam in order to receive the radiation. This method has several drawbacks. The scan time is long, which is uncomfortable for the patient and increases the likelihood of image discontinuity due to patient movement during the procedure (including involuntary motions such as breathing and heartbeats), adversely affecting the image quality. The patient may be exposed to radiation longer than necessary for imaging, which is undesirable. The mechanical devices necessary to maintain the beam/CCD array synchronization is complex and requires a special support system. Therefore, this device cannot be used interchangeably with standard film/screen mammography machines.
Also, the x-ray tube is on for a prolonged time period, and is subjected to high heat loading. A molybdenum x-ray target typically used in mammography will melt under this high heat load. To overcome this heat loading problem, a tungsten x-ray target is used. Tungsten targets operate at suboptimal high kV ranges (near 40 kVp), which detrimentally affects the contrast between adipose (fatty), glandular, or calcified breast tissue. As noted above, a low image contrast is unsuitable for imaging soft tissue such as breasts.
The second system is a multiple exposure system. In a multiple exposure system, the electronic imagers are positioned over various locations of the breast and then exposed to radiation at each location. This is repeated until the entire breast is imaged. This method also has several drawbacks. The mechanics of this device preclude it from being used interchangeably with standard film/screen mammography machines. To insure that the entire breast is imaged, some overlap between exposed locations occurs. The overlapped areas are exposed to twice as much radiation as in a standard mammogram. This method also has the disadvantage that the result is several images, not a single image, of the breast. Complicated image processing and registration techniques, such as "stitching", are required to view the image.
Therefore, it is an object of the present invention to provide a digital mammography machine that can image an entire breast in a single, low radiation exposure.
It is another object of the present invention to provide a digital mammography machine that can be used in existing film/screen mammography machines, without significant alteration to the machine, or complex mechanical devices.
It is a further object of the present invention to provide a digital mammography machine that uses conventional x-ray targets and optimal energy levels.
It is an even further object of the invention to provide a digital mammography machine which provides improved image quality, allowing clearer images of patients having breast implants.
It is yet over another object of the present invention to provide a digital mammography machine having sufficient contrast to distinguish between microcalcifications and surrounding breast tissue.
It is yet another object of the present invention to provide a digital mammography machine that allows a single technician to control a plurality of mammography machines from a remote control station.
It is yet an even further object of the invention to provide a digital mammography machine that allows the image to be transmitted over telephone lines or other communication channels.